Nuclear fuel rod supporting arrangement

ABSTRACT

A grid structure for holding a plurality of nuclear fuel rods. The grid structure is of the type having wall means, including rigidly interconnected generally rectangular metal strips, forming a plurality of passageways and adapted to support nuclear fuel rods within some of the passageways. The improvement comprises providing elongated slots intermediate and normal to the longitudinal edges of each of the strips at each intersection of the strips whereby the slots form openings in each corner of each passageway.

This is a division of application Ser. No. 105,388, filed Jan. 11, 1971now U.S. Pat. No. 3,933,583, which is a division of U.S. applicationSer. No. 774,148 filed Nov. 7, 1968, now U.S. Pat. No. 3,665,586.

A nuclear chain reaction is obtained by bombarding fissionable fuelmaterial with neutrons from a source of neutrons to split some of theatomic nuclei of the fuel into fragments, thereby releasing usefulenergy in the form of heat and other neutrons for bombarding othernuclei, and so on, ad infinitum, so long as each fuel nucleus undergoingfragmentation produces a minimum of one neutron on the average which inturn fragments another fuel nucleus. The process, called fission,depends for its continuity on an adequate supply of neutrons and fuel.To control the reaction, devices having large neutron absorptioncross-sections are generally utilized in conjunction with a moderatingmaterial for slowing down neutrons so that they possess the desiredenergy spectrum. The fuel, neutron absorbing material and moderator,together with associated structural components make up the core of thereactor, through which a heat transfer fluid is circulated to removeheat generated by the fission process.

The present invention is concerned with the associated structuralcomponents of the core, in that it is concerned with apparatus forspacing and supporting nuclear fuel bearing members in an uprightposition within the core of a nuclear reactor.

To achieve optimum power distribution across the core, considerationmust be given to the shape and distribution of the fuel bearing members.Since the heat generated by the fuel is more efficiently dissipated fromthe area surrounding the fuel when the fuel bearing member has a highratio of surface area to volume, modern day reactors utilize a pluralityof slender elongated fuel members or rods which are each made of alength of fuel material enclosed in a relatively thin-walled circulartube of cladding material. A number of fuel rods are clustered togetherin a composite assembly known as a fuel element, whereas a plurality offuel elements make up the core. Due to their slender construction,whatever means is used to space and position the fuel rods relative toone another must have good mechanical stability as well as the abilityto compensate for the slight differences in cross-sectional dimension ofthe individual fuel rods due to manufacturing tolerances. In addition,to expose the greatest possible surface area of a given rod to coolantflow; contact between the supporting structure and the rod, andobstructions to coolant flow around the rods, should be minimized.

To answer these needs, a variety of grid-like fuel rod spacing andsupporting arrangements have been developed. For example, U.S. Pat. No.3,225,090 issued to R. M. Leirvik, June 7, 1966, describes a fuelelement spacer grid having a plurality of open-ended tubular supportingsections adapted to have fuel rods inserted therein. In a preferredembodiment, each tubular section has an indentation formed in each ofits resilient side walls. The indentations extend towards the axis ofthe section to grip the fuel rod therewithin and firmly hold it inplace. Primary coolant freely bathes the region of the fuel rod which issurrounded by the walls of the section, since an elongated annular flowspace exists between the rod and walls except at the points where therod is contacted by the indentations. Since the walls are resilient,they compensate for slight variations in diameter of the fuel rods andlaterally impress forces on the fuel rods of sufficient magnitude tosupport the rods in an upright position within the core.

To assemble such prior art devices a fuel rod is either pushed or pulledthrough and into position within its respective spacer grid section. Thefuel rod itself is used to deflect a resilient means such as theresilient walls of Leirvik's tubular sections, resilient projectionsextending from rigid walls, resilient indentations or like resilientmovable fuel rod gripping members out of the passageway formed by thespacer grid arrangement to allow passage of the rod therethrough andinto position therewithin. Forces of up to 400 lbs. may be required toforce a fuel rod into position within the grid. Since highly specializedand elaborate loading equipment is usually required to position the rodswithin the grids, fuel elements are usually shop-assembled and shippedas units to the reactor site. The assembly process is not only timeconsuming and costly but results in fretting the fuel rod cladding,thereby affecting the integrity of the fuel rods.

Accordingly, there has been a long felt need for a more simplified gridstructure which can be readily loaded with fuel rods at the reactor sitewithout employing elaborate equipment, and a process for assembling fuelrods with grids which eliminates the fuel rod fretting problem. Thepresent invention provides a simply constructed spacer grid havingresilient holding means and provision for deflecting the holding meansout of the path of the fuel rod while the rod is being moved within thepassageway formed by the grid, such deflection being required wheneverfuel rods are either being loaded into the grid or unloaded from thegrid. The arrangement eliminates both the fretting problem and the needfor providing elaborate assembly equipment.

Accordingly, the invention is provided in combination with a frame forholding a movable nuclear fuel bearing member in place within a nuclearreactor wherein the frame comprises at least one open-ended cell whichforms a passageway within which the member may be held and resilientmeans for holding the member within the passageway and is an improvementcomprising means other than the member for temporarily enlarging thepassageway to enable the member to be freely moved within thepassageway. The preferred enlarging means comprises a means fortemporarily deflecting the resilient means associated with the cellularstructure. In greater detail, a plurality of rectangularly shaped panelsare provided to form the aforementioned cell. Preferably, at least oneof the panels acts as the resilient means and has a portion thereofformed to extend inwardly of the axis of the passageway to hinder motionof the fuel bearing member within the passageway, and the deflectingmeans is actuatable to move the panel portion outwardly of the axis ofthe passageway to enlarge the same thereby enabling the fuel bearingmember to be moved within the passageway without hindrance. Thedeflecting means is preferably at least one rotatable elongated elementwhich is endwise insertable within the cell and rotatable in placetherewithin to deflect the resilient wall, after which it may be againrotated and removed.

The invention also provides a method of assembling a nuclear fuel rodwithin a grid structure of the type which has resilient means for firmlysupporting the fuel rod within a nuclear reactor, wherein theimprovement comprises, temporarily deflecting the resilient means toallow the fuel rod to be moved within the grid structure withoutfretting or marring the fuel rod cladding. It should be appreciated thatthe method preferably includes the steps of deflecting the resilientmeans to move the rod into a predetermined position within the gridstructure without its being fretted thereby, and then releasing theresilient means to allow it to return and firmly hold the rod inposition. Assuming a rod is already in position within the gridstructure the method includes the steps of deflecting the resilientmeans, removing the rod, and releasing the resilient means; ordeflecting the resilient means, removing the rod, moving another rodinto position, and then releasing the resilient means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially exploded isometric view of a representativeportion of a grid structure according to the invention, showing anintermediate stage of assembly of a plurality of grid members;

FIG. 2 is a side elevation view of an assembled portion of a gridstructure;

FIG. 3 is a plan view of a representative portion of an assembled gridstructure with fuel rods and deflecting elements added;

FIG. 4 is a side elevation of a representative portion of a grid memberaccording to the invention;

FIG. 5 is a sectional view of the grid member of FIG. 4, takensubstantially along the line 5--5 of FIG. 4.

FIG. 6 is a side elevation of a representative portion of another gridmember according to the invention;

FIG. 7 is a pictorial side elevation of a fuel rod, showing an idealizedarrangement of the forces laterally imposed on a fuel rod by the gridstructure of FIG. 3 to support the rod in an upright positiontherewithin.

FIG. 8 is a plan view of FIG. 7;

FIG. 9 is a view of a typical deflecting means according to theinvention;

FIG. 10 is a partial side elevation view of the grid structure of FIG. 1showing a fuel rod and deflecting means in place therewithin takensubstantially along the line 10--10 of FIG. 3;

FIG. 11 is an enlarged portion of FIG. 3 illustrating the position ofthe deflecting means within a cell and before rotation thereof; and

FIG. 12 is a view similar to FIG. 11 illustrating the position of thedeflecting means after rotation thereof, taken substantially along theline 12--12 of FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings wherein like reference numerals designatelike or corresponding parts throughout the several views, there is shownin FIGS. 1-3 a fuel element spacer grid 10 for supporting a plurality ofnuclear reactor fuel bearing rods 12 in spaced parallel relationshipwith respect to one another. The fuel element spacer grid 10 is amulticellular metal frame composed of a plurality of substantially flatelongated members 14 which are made from a weldable and somewhatresilient metal that is able to survive containment within a nuclearreactor without significant alteration of its characteristic ofresiliency. The fuel bearing rods 12 are made of a nuclear fuel materialencapsulated in a thin-walled slender elongated sheath of a metalcladding which has a coefficient of expansion that is substantially thesame as that of the metal of which the frame is constructed therebyeliminating differential thermal expansion between the frame 10 and fuelrods 12 to eliminate a common cause of fretting and/or scratching therod cladding and/or damaging the frame during normal reactor operation.

In the preferred embodiment (FIGS. 4 and 5), each of the grid members 14is of the same construction to acilitate ease of fabrication of theframe. The preferred grid member 14 is a strip or band of metal havingopposing longitudinally extending faces and edges respectivelydesignated 15 and 16. A plurality of small like-dimensioned slots 18,formed in each of the edges 16, are substantially equidistantly spacedapart from one another along each of the edges 16, each slot in eachedge recessing a portion 17 of the edge 16 of the member and beinglocated directly opposite a corresponding slot in the opposite edge. Forexample, slot 18A₁ is located directly opposite slot 18B₁, whereas slot18A₂ is located opposite slot 18B₂, and so on, each slot 18A_(n) beinglocated opposite a corresponding slot 18B_(n), where n is an integer. Tosimplify the discussion, any portion of the grid member 14 which isbounded by centerlines 19 drawn through adjacent pairs of opposing slots18A_(n) - 18B_(n) and 18A_(n+1) - 18B_(n+1), where n is a particularinteger, will hereinafter be referred to as panel 20. A grid member 14may thus be described as being made up of a string of generallyrectangularly shaped panels 20.

In addition to the slots hereinbefore described, a string oflike-dimensioned and substantially equidistantly spaced paddle-shapedcutouts 22 are provided longitudinally of the length of the grid member14. The number of cutouts provided is equal to the number of pairs ofopposing slots 18A_(n) - 18B_(n) provided. Preferably, each cutout isaxially aligned with one of the aforesaid centerlines 19 and includes anarrow slit 24 which extends from the recessed portion 17 of slot 18towards the longitudinally extending axis of the grid member 14 where itis terminated by a symmetrically shaped opening 26 whose axis ispreferably coincident with that of the grid member. As will hereinafterbe described in greater detail, the slits 24 act as a means forinterlocking a given grid member with at least one other grid member;the slots 18 provide a recess within which weld metal is deposited afterinterlocking the grid members, and the openings 26 serve as accessopenings into an assembled multicellular spacer grid 10.

The preferred grid member 14 is also provided with a plurality ofindentations 28 formed therein at spaced intervals throughout its lengthand breadth such that each panel 20 is provided with three indentations28 aligned with one another and equidistantly spaced apart from oneanother on a centerline drawn through the panel parallel to theaforementioned centerlines 19. As shown in FIG. 1, each indentationangularly projects a localized portion of the grid member 14 laterallyof its length and breadth. Preferably, two of the indentations 28A and28B in a given panel 20, each project a portion of one of the freeopposing edges 16 of the panel in one direction of the length andbreadth of the panel, and the other indentation 28C locally projects thecenter of the panel in the opposite direction. The projections arethemselves rigid as compared to the resiliency of the grid member andrelatively stiff panels. The peak 32 of each projection 28 describes aline segment oriented perpendicular to the longitudinally extending axisof the grid member 14. As hereinbefore indicated the grid members areformed from a resilient strip of metal. In the preferred embodiment, agrid member is considered to have sufficient resiliency if at least oneof its panels 20 will freely return to its previous shape after itscenter is flexed relative to the edges of the grid member and in thedirection of projection of indentations 28A and 28B of the same panel.

A plurality of grid members 14 as hereinbefore described are preferablyassembled as shown in FIGS. 1-3. As best shown in FIG. 1, any two gridmembers are interlocked with one another by facewise orienting them atright angles with respect to one another such that the edges of themembers having slits 24 extending from slots 18 are similarly orientedand adjacent to one another; aligning one of the centerlines 19 drawnthrough a pair of opposing slots in one member, with a like centerlinein the other member; and endwise moving the members toward one anotherwhile they are so oriented and aligned until the opposing edges 16 ofthe members 14 lie in opposing parallel planes. Upon interlocking themembers it is seen that a slot 18A in one member is superjacent a slot18B in the other member, the spaces defined thereby intersecting oneanother at right angles to define opposing recesses having a basesdefined by intersecting recessed portions 17 of the edges 16 of eachmember 14. A plurality of grid members may be simultaneously interlockedwith one another in the same manner without departing from the spiritand scope of the invention. In any event, additional grid members may beadded to previously interlocked members in the same manner to form agrid structure 10 having at least one cell 35 and preferably a pluralityof cells 35 formed by a plurality of panels 20, wherein each of thepanels 20 acts as a side of at least one cell 35, and at least oneindentation 28 in each panel 20 projects inwardly of a cell. The gridstructure thus formed comprises a plurality of open-ended cells 35arranged in parallel rows to define a rectangularly shaped array ofcells terminated by outer grid members 14D, 14E, 14F, and 14G. Tocomplete the assembly of grid structure 10, a bead of weld metal 36 islocally deposited in each of the recesses to rigidly join the edges 16of the members together. More particularly, the bead 36 is deposited ineach of the recesses to join only the intersecting portions 17 of theedges 16 together.

In another embodiment of the invention, assuming all other particularsremain the same, the slots 18 may be eliminated and only the paddleshaped cutouts 22 and indentations 28 provided in each of the gridmembers 14. As shown in FIG. 6, the slits 24 would then be extended toan edge 16 of the member 14. Assembly of the grid structure 10 would beaccomplished in the same manner, terminating in the step of locallydepositing weld metal on the edges 16 of the members at their points ofintersection with one another to rigidly join the members together.

As shown in FIG. 3, a typical fuel rod 12 is supported in place in apredetermined position within its respective cell by a plurality ofindentations, the lateral forces imposed on a given fuel rod by opposingpanels 20 via their indentations 28 being sufficient to support the rodin place and in an upright position within the cell. In either of theembodiments there are six indentations 28 projecting inwardly of eachcell 35. The indentations formed in each pair of parallel spacedopposing panels of a given cell 35 provide three projections extendinginwardly of the cell. These indentations are equidistantly spaced apartfrom one another along the longitudinal length of the cell, one of theopposing panels providing two of the projections and the other providingone projection. Accordingly, two panels of a given cell each provide asingle projection extending inwardly of the same cell. Because of thisarrangement, the panels providing the same number of projections in thegiven cell are located at right angles and adjacent to one anotherrather than being located parallel and opposite to one another.

In FIG. 3, the inwardly extending indentations of the cells are referredto a common plane. In the embodiment shown, the minimum transversedimension "d" of an elongated straight passageway defined by the panelsof a given cell and extending endwise of the cell, is defined by theshortest straight line that can be drawn through the axis of the celland between indentations of opposing panels as referred to the commonplane. An elongated straight fuel rod having a minimum transversedimension slightly greater than the dimension "d" cannot be movedendwise through a cell and positioned therewithin unless the minimumtransverse dimension of the passageway is enlarged. In prior art devicesan end of the fuel rod is inserted within a cell and used as a wedge orlever to force one or more resilient members such one or more asresilient panels, resilient panels having projections, resilientprojections, resilient indentations, or like resilient movable fuel rodgripping means outwardly of the axis of the cell to enlarge thepassageway therethrough to permit the passage of an end of the rodthrough the cell for positioning the rod therewithin. As a consequence,the finely finished exterior surface of the fuel rods are fretted,scored, scratched, grooved or otherwise marred by the resilient means asthe rods are forcibly moved into a predetermined position within theirrespective cells. The rods are thereafter additionally fretted, scored,etc., when removed from their respective cells. Accordingly, it is afeature of the invention to provide means for temporarily enlarging thepassageway to enable the rod to be freely moved within the passagewaythereby eliminating the fretting problem, the enlarging means being ameans other than the fuel rod.

In the preferred embodiments hereinbefore described in detail, all ofthe panels act as resilient holding means for at least one fuel rodexcept the strings of panels making up two of the outside grid members(members 14D and 14E) since projections 28C of these members projectoutwardly of the grid structure 10. However, only two of the panels of agiven cell act as resilient holding means for the fuel rod associatedwith any particular cell, i.e., the adjacent panels which each have asingle indentation 28C protruding inwardly of a particularly cell. Eachof the panels which has a single indentation protruding inwardly of aparticular cell laterally forces the fuel rod contacted thereby towardsits opposing panel and jams it against the indentations 28A and 28Bthereof which protrude into the same cell. Indentations 28A and 28B arenot intended to be movable. The edges 16 of the grid members 14 arerigidly connected to one another at their points of intersection and theindentations 28A and 28B are themselves rigid projections. Ashereinbefore indicated, the central portion of a given panel isresiliently movable with respect to the edges 16 of the grid member 14and is movable outwardly of the cell with which indentation 28C isassociated and in the direction of projection of the other indentationsof the same panel. When moved, the center of the panel carries the rigidcentrally located indentation 28C therewith to enlarge the passagewayfrom which the peak 32 of the indentation 28C is withdrawn. If thecentral portions of both panels having single indentation protrudinginwardly of the same cell are both deflected outwardly of the cell, thepassageway is enlarged to a greater extent than if only one of thepanels is so deflected. Accordingly, the means for temporarily enlargingthe passageway when desired comprises; deflecting means for moving theresilient fuel rod gripping means outwardly of the axis of the cell. Thedeflecting means preferably includes a means for actuating thedeflecting means, but this feature is not essential.

Assuming a fuel rod is to be positioned within a cell, the deflectingmeans is actuated to temporarily outwardly deflect the panels having asingle indentation protruding inwardly of the cell, to allow the rod tobe freely moved within the cell's passageway by means well known in theart. The panels need only be deflected an amount necessary to permit therequisite portion of the fuel rod to be freely passed through thepassageway for proper positioning therein. The word "freely", as used inthis disclosure, characterizes the movement of a fuel rod within itsrespective cell or passageway and is intended to describe movement"without restraint", "without hindrance" or "without restriction" withinthe cell or passageway; or "without interplay" between the fuel rod andresilient means when the rod is moved within the cell, i.e., when it iseither moved into position within the cell or passageway or removedtherefrom. When the rod is so "freely" moved, it might possibly be movedwithout even being contacted by the indentations 28A, 28B and 28C. Inany event, movement of the rod is accomplished without fretting theexterior surface of the rod. After the rod is so positioned within thepassageway the deflecting means is actuated to release the resilientmeans thereby allowing the panels to resiliently return, carry theindentations 28C inwardly of the passageway and laterally jam the fuelrod against indentations 28A and 28B in the opposing panels. Assuming arod is to be removed from a cell, the procedure is the same; the panelsare deflected, the rod removed, and the panels released.

Assuming a rod is being held within the passageway, the laterallydirected forces impressed on the rod by the indentations 28A and 28C tosupport the rod and retain it in place within the cell, are distributedalong the longitudinal length of the rod as shown in FIGS. 7 and 8. Forthe sake of simplicity the reference numeral identifying each of theforces shown in FIGS. 7 and 8 corresponds to the reference numeral ofthe indentation which imposes the particular force on the fuel rod. Thusforce vectors 28C are each imposed on the fuel rod by a panel via itsindentations 28C whereas each pair of force vectors 28A and 28B, located180° removed from one of the vectors 28C, is imposed on the fuel rod bya panel via its indentations 28A and 28B. It should be noted that thepairs of force vectors 28A and 28B do not oppose one another; they aredisplaced from one another less than 180°. If the forces 28C are, on theone hand, sufficiently reduced in magnitude, or, on the other hand,entirely eliminated, the fuel rod could be easily moved within the cell.The preferred deflecting means is adapted to so reducing or eliminatingforces 28C thereby allowing the fuel rod to be freely moved within thecell without being fretted by the indentations.

As shown in FIGS. 9-12, the preferred deflecting means, for theembodiments shown, comprises two elongated elements 40 such as a pair ofrigid shafts, rods, bars, tubes or the like, with provision for movingthe resilient means of a grid structure having at least one cell toenlarge the fuel rod passageway of the cell, thereby allowing a fuel rodto be freely moved in the passageway without being fretted. Each element40 has a portion 42 of its length offset from its axis of rotation toact as a cam for moving resilient means, and is terminated at one of itsends with an actuating means 44 which preferably comprises a lever armextending from the element at right angles to its axis. The other end ofthe element is hereinafter referred to as the free end thereof.

As shown in FIG. 2, when the grid structure is assembled, the openings26 in the grid members 14 are aligned with one another to define a pairof open channels 45 associated with each grid member and extendinglongitudinally of the length thereof. Each channel, in a given pair ofchannels extends through the grid structure parallel to thelongitudinally extending axis of the grid member with which it isassociated. One channel 45A of each pair of channels 45 isintermittently obstructed throughout its longitudinal length by thecentrally located indentations 28C of the grid member with which thechannel pair is associated. Only one channel of each pair is soobstructed since all of the centrally located indentations 28C in anygiven grid member project in the same direction laterally of the lengthand breadth of the grid member. In order to move a fuel rod within aparticular cell the grid members which have a centrally locatedindentations 28C extending inwardly of that cell must first beidentified. Upon identification of each of the particular grid members,channels 45A associated therewith are readily located. The free end ofthe element 40 is inserted into the grid structure through either of theparticular channels 45A so as to position the offset portion 42 of theelement in the desired cell. When inserting the element 40, care must betaken to orient the offset portion 42 of the element in a planeapproximately parallel to the plane of the grid member associated withthe element. The element is inserted into the channel 45A until itsoffset portion is located next adjacent to the indentation 28C inwardlyof the desired cell. The element is then actuated by means well known inthe art to cause the element to be rotated one-quarter of a turn eitherclockwise or counter-clockwise, but in any event, in the direction whichcarries the passageway, thereby flexing the cell panel havingindentation 28C outwardly of the passageway. The positions of theindentation 28C, before and after rotating the element 40, arerespectively shown in FIGS. 11 and 12. The same procedure is used tomove the other inwardly extending indentation 28C outwardly of thedesired cell, another element 40 being inserted into the appropriatechannel 45A extending at right angles to the first channel 45A, as shownin FIGS. 11 and 12.

In view of the foregoing it should be appreciated that othermulticellular grid structures having resilient means associatedtherewith may be provided wherein each cell has only one resilient panelfor holding the nuclear fuel bearing member therewithin. For example, agiven grid structure may have at least one cell formed by threeconnecting panels, rather than four, to form a triangularly shaped cellwith one panel acting as the resilient means for holding a nuclear fuelbearing member therewithin, the resilient panel being adapted to jam therod against the other panels or protrusions therefrom. In this case, asingle deflecting element 40 may be utilized as the deflecting means forenlarging the passageway through the cell. Accordingly, it is a featureof the invention to provide at least one rotatable deflecting element 40endwise insertable within a grid structure having at least one celldefined by a plurality of panels, wherein only one of the panels of aprojection therefrom acts as the resilient means allowing the fuel rodto be freely moved within the passageway defined by the cell. Inaddition to the foregoing, it should be appreciated that one or moredeflecting elements may be provided with a plurality of offset portions42 distributed along the longitudinal length of the element forsimultaneously deflecting the resilient means associated with more thanone cell. In this case, the centers of adjacent offset portions may bespaced apart from one another a distance equal to or a multiple of thedistance between centers of adjacent panels in the same grid member.Upon rotation of the deflecting element each of the resilient panelswith which an offset portion is associated will be deflected outwardlyof the cell with which the resilient panel is associated to allow aplurality of fuel bearing members to be freely moved within theirrespective cells.

What is claimed is:
 1. A member used within a nuclear reactor fuelelement spacer grid comprising a generally planar strip havinglongitudinally extending faces on opposite sides of said strip and edgesgenerally transverse to said faces, said strip having a plurality ofslots formed therein that intersect and are generally perpendicular toat least one of said edges, said edges being interrupted at equallyspaced intervals by said slots, each of said slots being longitudinallyaligned with a corresponding slot in the opposite edge, a plurality oflongitudinally aligned narrow slits each respectively terminating in awider symmetrically shaped opening to form paddle-shaped cutouts thatare situated essentially in the middle of said strip and intermediate ofsaid strip edges, each of said slots of one of said edges terminating inone of said narrow slits longitudinally aligned therewith, a pluralityof indentations formed in strip portions that are generally intermediateof said paddle-shaped cutouts, said indentations being formed atequidistantly spaced intervals throughout the length and breadth of saidstrip in the edges of one face of said strip portion and in the centerof the opposite face of said strip portion so as to protrude beyond thestrip plane.